A comparator is an electrical circuit that provides an output electrical signal which is a function of the difference in magnitudes of electrical signals applied respectively to a plurality of circuit input terminals. A typical comparator may, for example, comprise an output terminal, first and second input terminals, and an electrical circuit connecting said output and input terminals to provide a "hi" or "lo" voltage at the output terminal whenever the voltage at the first input terminal equals or exceeds the voltage at the second input terminal, and a "lo" or "hi" voltage at the output terminal in all other cases. The term "offset comparator" refers to a species of comparators wherein the shift from "hi" to "lo" (or "lo" to "hi") occurs when one of the input voltages (or currents) equals or exceeds the other by a pre-established threshold reference voltage (or current) amount. The term input "common mode" voltage (or current) refers to the average of the voltages (or currents) present at the input terminals. Because of intentional and/or unintentional fluctuations in input voltages (or currents), comparators are designed to work over some given range of common mode voltages (or currents). The term "voltage" used in reference to input and output voltages hereafter will be understood to cover input and output voltages, or alternatively input and output currents.
There is a problem with conventional offset designs in maintaining a desired offset voltage over a wide range of input common mode voltages. This is because conventional offset reference voltages are set without being tied directly to the input voltages themselves. Ideally, the offset values should remain constant despite fluctuations in input common mode voltage. However, conventional offset voltage circuits behave nonlinearly, so are subject to deviations due to variations in input common mode voltages.